1. Technical Field
The present invention relates to a method and a device for stabilizing a vehicle including a trailer drawn by the towing vehicle, wherein the vehicle is monitored in terms of snaking motions, and upon detection of an unstable snaking motion, a moment essentially damping the snaking motion is applied to the vehicle.
2. Description of Related Art
A vehicle of this type including a trailer (car-trailer combination) inheres the risk that quick steering movements, side wind or road irregularities can induce oscillations and cause rolling of the vehicle. Depending on the driving speed, the oscillations can decay, remain constant, or increase (undamped oscillation). When the oscillations remain constant, the car-trailer combination has reached the critical velocity. Above this speed threshold, a car-trailer combination is unstable, below said threshold it is stable, that means, possible oscillations die out. The magnitude of this critical speed depends on the geometry data, the tire rigidities, the weight and the distribution of weight of the towing vehicle and the trailer. Further, the critical speed is lower in a braked driving maneuver than at constant travel. In turn, it is higher during accelerated driving than at constant travel. Methods for stabilizing car-trailer combinations are known in various designs. DE 199 64 048 A1 discloses that upon the detection of a snaking motion, a yaw torque is automatically applied to the vehicle, which is basically in antiphase to the snaking motion. The snaking motion is detected by evaluation of the measured lateral acceleration, and the velocity and/or the steering angle of the vehicle are taken into consideration in order to distinguish a snaking motion from steering movements of the vehicle. The brake pressures shall be determined depending on the frequency, the phase of the snaking motion.
Further, the objective of DE 100 32 230 A1 is to determine the yaw motion of the vehicle and to detect the phase of the oscillation at any time by means of an oscillation analysis (Fourier transformation). This method, which manages without sensing the steering angle, provides to intervene into the left-hand or right-hand brake of the vehicle by means of a short, abrupt pulse in the correct phase, in order to damp the oscillation.
According to EP 0 765 787 B1, the amplitude of the lateral acceleration or the yaw angle velocity (yaw rate) is taken into consideration, which shall lie within a predefined frequency band and exceed a predefined threshold value, in order to introduce brake pressure into the brake devices of the wheels of the vehicle. In this arrangement, the condition that a steering motion quantity remains under a predetermined threshold shall be complied with.
Further, EP 0 989 049 A2 describes a car-trailer combination and a method changing the steering angle of the wheels of a vehicle axle of the tractor upon the occurrence of a difference value of the actual and nominal yaw motions of the towing vehicle in order to reduce the difference. It is preferred that the steering angle of all wheels of the steerable vehicle axle is changed by the same amount.
Generic DE 100 48 418 A1, on the other hand, again arranges for damping an analyzed oscillation by a periodically varying yaw torque, which is generated by reciprocal brake intervention of an ESP control on the vehicle. Corresponding to DE 100 32 230 A1, the Fourier transformation is used to calculate the amplitude of the yaw angle deviation and to activate the brake intervention when a threshold value is exceeded. As this occurs, the periodically varying yaw torque is generated with a phase delay in relation to the sensed vehicle oscillating motion, and it has a frequency, which corresponds to the oscillation frequency of the sensed periodic vehicle oscillation frequency. This is required because an ESP control comprises a brake intervention, which in the case of driving stabilization upon understeering or oversteering of the vehicle, however, does not intervene with a sufficient rate of sensitivity, on the one hand, and does not take place at the optimal point of time, on the other hand.
The known methods perform an oscillation analysis, for what purpose great calculating effort is needed. The great calculating effort needed increases the costs of operation considerably. Generic DE 100 48 418 A1 provides a yaw torque to be applied to the vehicle, which is reduced proportionally to the decreasing amplitude of the damped vehicle oscillation. Upon detection of the vehicle oscillation and application of the yaw torque, all methods are based on a periodic oscillation, which shall be damped by means of a yaw torque in antiphase and which stays periodical and decreases after damping. Surprisingly, however, it has shown that an application of a counteracting moment will not dampen the oscillation of the vehicle with trailer, but only changes the course of oscillation, i.e. the oscillation becomes disturbed and is destructed completely.